(1) Field of the Invention
The present invention relates generally to an automotive vehicular suspension. More specifically, the present invention relates to a system and method for variably controlling a damping force coefficient of a shock absorber (hereinafter referred to as a damper) installed between an unsprung mass and a sprung mass of the automotive vehicle.
(2) Description of the Background Art
A Japanese Patent Application First (unexamined) Publication No. Showa 64-60411 published on Mar. 7, 1989 exemplifies a previously proposed damping force coefficient controlling system for a damping force variable damper.
This previously proposed-damping force coefficient controlling system detects a relative speed of an unsprung mass with respect to a sprung mass, compares the detected relative speed (damping force) with a predetermined threshold value, and controls the damping force coefficient toward a high damping force coefficient side when the relative speed exceeds the threshold value.
However, since at a high frequency region in which an sprung mass vibrating frequency exceeds a resonance frequency, a frequency (number of times) at which the relative speed (damping force) exceeds the predetermined threshold value, the damper is held at the higher damping force coefficient side. Therefore, the damping force which is generated is more than necessary and results in vehicular comfort being degraded.
In addition, another previously proposed damping coefficient variably controlling system has been disclosed in a Japanese Patent Application No. Showa 61-163011 published on Jun. 23, 1986.
This second previously proposed damping force coefficient controlling system derives the sprung mass speed (velocity) and the relative speed (velocity) between the unsprung mass and sprung mass, controls the damping force coefficient toward the higher damping force coefficient side when a sign (polarity) of the sprung mass speed coincides with a sign (polarity) of the relative speed, and controls the damping force coefficient toward a lower damping coefficient side when the polarities do not coincide.
However, although there is no problem in a case where the sprung mass vibrating frequency occurs at a frequency lower than the sprung mass resonance frequency, the control timing deviates from its normal timing due to influences of control electrical delay and hydraulic response delay of hydraulic pressure in a hydraulic chamber of the actual damper in a case where a continuous road surface input exceeding a predetermined dead frequency at which an sprung mass transmissibility would receive no effect from the set damping force coefficient on the damping force coefficient is present. Consequently, the vehicular comfort is worsened.